1999A&A...345..233O

We present the result of a quantitative analysis of ¹²C/¹³C ratios in 26 J-type carbon stars. The ¹²C/¹³C ratios are determined from lines of the CN red system around 8000Å, using the iso-intensity method and line-blanketed model atmospheres. The average of ¹²C/¹³C ratios in the 26 stars is 4.7±2.8 (standard deviation). All the stars studied, except for two stars, have ¹²C/¹³C ratios smaller than 10. ¹²C/¹³C ratios as low as 1∼2, which are lower than the value at the equilibrium of the CN-cycle, are found for a significant fraction of our sample, suggesting the operation of non-equilibrium nuclear processes. For several stars previously analyzed by other authors, our result shows fair agreement. The serious disagreement of ¹²C/¹³C ratios, which we reported for N-type carbon stars in our preceding paper, is not found for J-type carbon stars. Five silicate carbon stars in our sample show no peculiar ¹²C/¹³C ratios among the stars studied in the present work. This result implies that the mechanism responsible for low ¹²C/¹³C ratios in silicate carbon stars might be the same with that operating in other J-type carbon stars. In other words, ¹²C/¹³C ratios in silicate carbon stars have turned out to give few clues to identify the mechanism responsible for their formation.